(1) Field of the Invention
The present invention relates to improvements in a piezoelectric fuel injector for use in spray-injecting injecting a metered amount of low-pressure fuel, such as gasoline, into an intake air stream drawn into the cylinders of a spark-ignition internal combustion engine. More particularly, the present invention relates to the fuel injector of the nature described which is operable at a high frequency in response to a high frequency drive signal in order to ensure precise metering of the quantity of fuel throughout a wide range of engine operating conditions.
(2) Description of the Related Art
It is well known to use piezoelectric actuators for pumping and metering fuel in fuel injectors. An example of the prior art piezoelectric fuel injectors is disclosed in U.S. Pat. No. 3,194,162 granted to A. L. W. Williams. This fuel injector is provided with a piezoelectric drive unit associated with a piston. The injector has an outlet passage which is in the form of a horn normally closed by a spring-biased pintle valve. When the piezoelectric drive unit is excited causing sudden movement of the piston, a shock wave is imparted to the fuel stored in the horn. As the shock wave advances in the horn, the fuel pressure is increased, causing the pintle valve to give way, thereby allowing a small quantity of fuel to be injected into the engine cylinder. Williams anticipates the use of his injectors for to diesel engines as well as spark-ignition engines.
Contemporary spark-ignition engines are often provided with electronically-controlled fuel injection systems equipped with low-pressure fuel injectors, as opposed to high-pressure fuel injector used in diesel engines. In spark-ignition engines, demands for fuel per unit time interval vary significantly from the minimum quantity required during engine idling up to about 50 times the minimum required during full-throttle high-speed operation. This variation in fuel demand is even greater in turbocharged engines The essential requirement for low-pressure fuel injectors is that they be capable of metering the quantity of fuel as precisely as possible over a wide range of engine operating conditions.
When a piezoelectric fuel injector such as that proposed by Williams is used in electronically-controlled fuel injection systems, metering of fuel may be achieved by controlling the voltage applied to the piezoelectric element, thereby varying the amount of expansion of the element in such a manner as to in turn vary the distance of travel of the piston and, hence, the displacement of the piston for each pumping stroke. Alternatively, or in combination with the voltage variation, the control of quantity of fuel may be effected by changing the number of actuations of the piezoelectric actuator per unit time interval as disclosed in a copending U.S. patent application Ser. No. 549,372 filed November 7, 1983 and assigned to the assignee of the present invention. In the latter case, the frequency of operation of the piezoelectric drive, i.e., the number of operations per unit time interval, must be varied from the minimum value to the maximum in order to meet the wide variation in the fuel demand mentioned above.
One of the disadvantages of conventional piezoelectric fuel injectors such as that of Williams is that the quantity of fuel injected per unit time interval is not increased in proportion to, or in a linear relationship with, the increase in the frequency of operation, particularly in the high-frequency operation of the piezoelectric drive unit, so that precise metering function is not achieved during high-speed operation. This is because the quantity of fuel injected per pumping stroke fluctuates considerably at the high-frequency operation.
Another disadvantage of the prior art piezoelectric fuel injectors is that the outlet passage leading from the pumping chamber must be made as a narrow opening for being closed by the pintle valve. The reduction in the cross-sectional area of the outlet necessarily entails that the fuel be injected in a relatively thick layer through the annular clearance between the outlet opening and the pintle valve in order to achieve the required amount of fuel injection. This tends to result in insufficient subdivision and vaporization of fuel droplets. This prevents even distribution of the air/fuel mixture over different cylinders, particularly when the fuel injection system is of the single-point type wherein only one injector is provided for supplying fuel for all cylinders.